The invention relates generally to hybrid and electric vehicles, and more specifically to systems for operating auxiliary systems aboard hybrid and electric vehicles.
Purely electric vehicles typically use stored electrical energy to power an electric motor, which propels the vehicle. Hybrid electric vehicles combine an internal combustion engine and an electric motor that is typically powered by one or more electrical energy storage devices. Such a combination may increase overall fuel efficiency by enabling the combustion engine and the electric motor to each operate in respective ranges of increased efficiency. Electric motors, for example, may be efficient at accelerating from a standing start, while combustion engines may be efficient during sustained periods of constant engine operation, such as in highway driving. Having an electric motor to boost initial acceleration allows combustion engines in hybrid vehicles to be smaller and more fuel efficient.
In conventional vehicles, auxiliary systems include air conditioning, power steering pumps, oil pumps, coolant fans and air compressors, and the like, and are typically driven by belts and gear drives powered by the vehicle's internal combustion engine. However, electric and hybrid vehicles generally power auxiliary systems using stored or recaptured electrical energy. In some cases, for example, such as a transit bus operating in an urban area, energy required to operate auxiliary system loads may exceed the energy needed to propel the vehicle. Efficiency improvements in the operation of auxiliary systems may increase the driving range of electric vehicles and may reduce fuel usage and tail pipe emissions in hybrid vehicles.
Auxiliary systems in hybrid and electric vehicles may be powered directly from a battery, fuel cell, or other energy storage device, or may be powered through the traction drive DC link. One auxiliary system may run most efficiently at a voltage different from that needed for efficient operation of another auxiliary system. However, one common system design may cause the auxiliary systems to run at a voltage provided by the fuel cell or battery supplying power thereto even if the provided voltage is not ideal for a specific auxiliary system. Moreover, the voltage provided by such a battery or fuel cell may vary widely with the load placed thereon. As a result, some auxiliary systems may waste power by operating inefficiently at widely varying or suboptimal voltages. It would therefore be desirable to have a system capable of supplying stable power to a plurality of auxiliary systems at a voltage where each system operates most efficiently.